Halliday And Matthiessen (2014: 675-6):
Just as the different domains of manifestation are textually distinct, they are also interpersonally distinct. When the domain of manifestation is a cohesive sequence of clauses, or a paratactic nexus of (free) clauses, the two figures related by expansion are enacted interpersonally as propositions or proposals. This means that each of them can be negotiated in their own right – accepted or denied, complied with or refused, and so on, as in she didn’t know the rules – oh yes, she certainly did. The same is true of the dominant (α) clause of a hypotactic nexus, since if it is a free clause, it realises a negotiable proposition or proposal. However, while the dependent (β) clause supports a proposition or proposal, it does not constitute one itself ; and if it is non-finite, it is even further removed from the realm of negotiation. A causal dependent clause (e.g. because she didn’t know the rules) is thus not presented as directly accessible to negotiation; it has to be accepted without argument.
When the domain of manifestation is a simple clause, there is just a single proposition rather than two. This obviously restricts the scope for negotiation; but when the causal relation is construed within the Process, it has become propositionalised or proposalised, and can be assessed … and negotiated … Here it is no longer the cause or the effect that is held up for negotiation but rather the causal relation. Being construed as nominal groups, the cause and the effect are not negotiable at all.